(1) Field of the Invention
The present invention relates to a process for the production of a double-oriented electrical steel sheet composed of crystal grains having &lt;001&gt; orientations of easy magnetization axes in the longitudinal direction of the steel sheet (strip) and the direction orthogonal to the longitudinal direction and having a {100} plane ({100}&lt;001&gt; expressed by Miller indices) appearing on the rolled face.
(2) Description of the Related Art
A grain-oriented electrical steel sheet which is especially magnetized (the flux density is high) in the rolling direction (longitudinal direction) of the steel sheet (strip) and has a small watt loss has been heretofore produced typically by the process disclosed in the specification of U.S. Pat. No. 1,965,559. This grain-oriented electrical steel sheet is composed of crystal grains having a {110} plane appearing on the rolled face and a &lt;001} orientation ({110}&lt;001&gt; expressed by Miller indices) as the easy magnetization axis only in the rolling direction (the longitudinal direction of the steel sheet).
Compared with the grain-oriented electrical steel sheet having excellent magnetic characteristics only in the rolling direction (for example, the B.sub.10 value in the rolling direction is 1.92 Tesla while the B.sub.10 value in the direction orthogonal to the rolling direction is 1.45 Tesla), a double-oriented electrical steel sheet is advantageously used as the iron core material for a large-size rotary machine because the double-oriented electrical steel sheet has easy magnetization axes in both the longitudinal direction of the steel sheet (strip) and the direction orthogonal to the longitudinal direction of the steel sheet (strip) and has excellent magnetic characteristics in both directions.
On the other hand, a cold-rolled, non-oriented electrical steel sheet in which the easy magnetization axis is not highly integrated is generally used in a small-size rotary machine. If the double-oriented electrical steel sheet is used in this type of machine, a reduction of the size of the machine and an increased efficiency can be very effectively obtained.
The double-oriented electrical steel sheet has superior magnetic characteristics compared to those of the grain-oriented electrical steel sheet, but the doubleoriented electrical steel sheet has not been manufactured as an industrial product.
As the bench-scale process for the production of a double-oriented electrical steel sheet, the following two processes have been published, but each involves problems as industrial-scale preparation processes.
One prior art process is disclosed in the specification of U.S. Pat. No. 3,130,095. In this process, the material is subjected to high-temperature annealing in an atmosphere containing a polar gas such as hydrogen sulfide to control the surface energy and selectively grow grains in the {100}&lt;001&gt; orientation.
In this process, however, it is necessary to strictly control the atmosphere on the surface of the steel sheet, and the process is not suitable for mass production.
The other prior art process is a process proposed by Satoru Taguchi et al in the specification of U.S. Pat. No. 3,163,564, in which the dispersed precipitates are controlled. This process is a cross-cold-rolling process in which first a cold rolling is performed in one direction and second a cold rolling is carried out in the direction orthogonal to the first rolling direction. In this cross cold rolling process, a method is adopted in which, after the first cold rolling, the strip is cut into a predetermined length to form a steel sheet and the steel sheet is subjected to the second cold rolling in the direction orthogonal to the first cold rolling direction, or a method in which the cut sheet is turned by 90.degree. so that both side edges of the strip subjected to the first cold rolling are welded to form a strip and the second rolling is subsequently performed.
For the manufacturer, these methods are complicated and it is difficult to obtain steel sheets having a uniform shape, and for the users, these methods are not satisfactory. More specifically, where the material is supplied in the form of a sheet, the efficiency of the punching operation is very low, and where the material is supplied in the form of a strip coil, welded parts appear at intervals, and since the magnetic property is insufficient thereat, the welded parts must be removed.
Accordingly, for the reasons mentioned above, double-oriented electrical steel sheets manufactured by the conventional techniques are not used as industrial products.
In addition to the above-mentioned problems, the technique disclosed in the specification of U.S. Pat. No. 3,163,564 involves a serious problem which prevents industrial use of the process. More specifically, according to this cross cold rolling process, a product having relatively high magnetization characteristics (B.sub.10 value) can be obtained, but these magnetization characteristics do not match the high manufacturing cost due to the complications of the preparation process. Therefore, the product is not superior to the conventional grain-oriented electrical steel sheet. Moreover, since the development of the technique disclosed in U.S. Pat. No. 3,159,511 (Japanese Examined Patent Publication No. 40-15644) and U.S. Pat. No. 3,932,234 (Japanese Examined Patent Publication No. 51-13469), the magnetization characteristics (B.sub.10 value) of the grain-oriented electrical steel sheet have been rapidly improved, and the requirement of B10.gtoreq.1.89 Tesla is stipulated in JIS, and products having a B.sub.10 value of about 1.92 Tesla are now marketed.
Under this background, the double-oriented electrical steel sheet should have magnetization characteristics comparable to those of the grain-oriented electrical steel sheet. As a means of improving the magnetization characteristics of the double-oriented steel sheet, Japanese Examined Patent Publication No. 38-8213 proposes a process in which a hot-rolled material is annealed and then cold-rolled in directions orthogonal to each other. But the magnetization characteristics obtained by this process are not satisfactory.
The iron core material should have excellent watt loss characteristics (small watt loss value, W/kg) as well as the above-mentioned magnetization characteristics. An increase of the B.sub.10 value and reduction of the thickness of the product are especially effective for improving the watt loss characteristics. In the field of grain-oriented electrical steel sheets, JIS stipulates that the thickness should be as thin as 0.23 mm, but in a steel sheet having such a small thickness, it is very difficult to obtain highly oriented {100}&lt;001&gt; grains. Even in the processes disclosed in the specification of U.S. Pat. No. 3,163,564 and Japanese Examined Patent Publication No. 38-8213, the final thickness attainable is 0.30 mm or more, and the B.sub.10 value of the obtained product is 1.85 Tesla at highest. As a means of eliminating this disadvantage, U.S. Pat. No. 3,136,666 (Japanese Examined Patent Publication No. 35-17208) proposes an improved technique, but in this improved technique, cold rolling and annealing are added, and therefore, the manufacturing cost is drastically increased.